1. Technical
The present invention relates to a light guide plate and a surface light source device, and specifically relates to a light guide plate, a surface light source device.
2. Related Art
In recent years, with a reduction in a thickness of a mobile device that has a surface light source device, a reduction in the thickness of the surface light source device is also being required. To reduce the thickness of the surface light source device, it becomes necessary to reduce a thickness of a light guide plate. However, even when the thickness of a planar light guide plate can be reduced, there is a limit to a reduction in a height of a light source composed by an LED. Therefore, when a thin planar light guide plate is used, the height of the light source becomes larger than a thickness of an end surface (a light-incident end surface) of the light guide plate, and the light source arranged opposite to the light-incident end surface of the light guide plate is protruded to above an upper surface of the light guide plate. When the light source is protruded to above the light guide plate, not all the light emitted from the light source is incident to the light-incident end surface, but a part of the light leaks to an outside, and light utilization efficiency becomes poor.
FIG. 1 is a perspective view schematically illustrating a conventional surface light source device. A surface light source device 11 in FIG. 1 is configured by a light source 12 and a light guide plate 13, and the light source 12 is arranged opposite to a light-incident end surface 19 of the light guide plate 13. The light guide plate 13 is provided by integrally forming a light guide-plate main body 15 that has substantially a uniform thickness and a light introducing portion 14 that has a wedge shape. An inclined surface 16 is formed on an upper surface of the light introducing portion 14, and a directional transform pattern 17 (light-leakage prevention patterns) formed of a plurality of V-grooves 18 is formed on the inclined surface 16. A thickness of an end surface (the light-incident end surface 19) of the light introducing portion 14 is larger than a height of the light source 12. As a surface light source device that uses a light guide plate having such a structure, there is a one disclosed in Japanese Unexamined Patent Publication No. 2008-16432, for example.
In the surface light source device 11, light emitted from the light source 12 is incident from the light-incident end surface 19 into the light introducing portion 14. Since the thickness of the end surface (the light-incident end surface 19) of the light introducing portion 14 is larger than the height of the light source 12, the light emitted from the light source 12 is efficiently taken into the light introducing portion 14. The light incident to the light introducing portion 14 is reflected by the upper surface (the inclined surface 16) or a lower surface of the light introducing portion 14, and is guided to the light guide-plate main body 15. When the light is emitted from a light emitting surface 20 of the light guide-plate main body 15, a directional characteristic is spread in a width direction by lenticular lenses 21.
In this case, when the inclined surface 16 is flat, there is a risk that a part of the light that is incident to the light introducing portion 14 leaks to an outside by being transmitted through the inclined surface 16, without being reflected by the inclined surface 16. Therefore, in the surface light source device 11, light leakage from the inclined surface 16 is reduced, by providing the directional transform pattern 17 on the inclined surface 16. That is, in the surface light source device 11, since the V-grooves 18 are provided on the inclined surface 16, an incidence angle of the light incident to the V-grooves 18 becomes large, light is totally reflected by the V-grooves 18, and light L is hard to leak from the inclined surface 16. As a result, in the surface light source device 11 in such a structure, it becomes possible to improve light utilization efficiency and reduce the thickness of the surface light source device.
However, in the surface light source device 11, since the light incident to a groove-inclined surface of the V-grooves 18 is totally reflected to a lateral direction relative to an incident direction, light that is guided to outward inclined directions increases, viewed from the light source 12. Therefore, emission lines are generated in outward inclined directions, viewed from the light source 12. When such emission lines are generated, the surface light source device 11 glitters brightly when the surface light source device 11 is looked at from inclined directions, and visibility becomes poor.
FIG. 2 is a simulation view of a state that emission lines a are generated in outward inclined directions in a sample of the surface light source device 11 (however, optical patterns such as deflection patterns are not provided on a lower surface of the light guide plate 13).